Four years ago, Valve's free to play MOBA Dota 2 hit 1,295,114 concurrent players and today it's finally been beaten by Counter-Strike: Global Offensive.
Why the sudden resurgence of CS:GO? Well, I've been following it along and noted a few times now about certain milestones both Steam and CS:GO have been hitting. Have a look at this previous article for a little timeline, it's absolutely crazy.
Looking over the numbers, currently at around 1,300,317 (SteamDB) I do believe this makes it Valve's most popular game when it comes to players online—ever.
You could certainly put it down to lots of people staying at home due to the Coronavirus. That's a fair point. However, I would say it's also partly due to Riot's upcoming FPS 'Valorant'. It looks a huge amount like CS:GO, seriously it's hard to find someone who says it doesn't look like it (they're right though), with some special abilities added in. There's been a lot of talk about this, and I've seen quite a lot of people talk about how they actually went back to CS:GO so I think that's actually playing a part too.
I must admit my surprise a little that Valve hasn't really tried to capitalize on the opportunity of it expanding again. Their recent Battle Pass with Operation Shattered Web was pretty well received (I enjoyed the co-op missions and a few Stars earned in it), so I was expecting another Battle Pass to be put out already since that finished on March 30.
You can find Counter-Strike: Global Offensive free to play on Steam.